1. Field of the Invention
The present invention pertains to the field of fiber-reinforced thermosetting composites. The invention pertains to fiber-reinforced prepregs containing thermosetting resin systems containing particulate elastomers having a T.sub.g of 25.degree. C. and below, and the composites prepared therefrom. In particular, the invention pertains to prepregs having crosslinked elastomer particles which improve the surface tack retention, thus providing for a prepreg system having improved tack quality.
2. Description of the Related Art
The use of fiber-reinforced thermoset composites continues to grow. While great strides have been made in increasing the strength, toughness, temperature use ceiling and other important physical properties, other improvement is still required. Although some resin systems, e.g., the bismaleimides, perform well in high temperatures, epoxy resin systems remain the system of choice for many applications. Cyanate ester systems are also preferred for some applications.
In practice, the lack of tack can be a severe limitation if the prepregs contain particles. This lack of tack has limited their use in many applications, especially in complex shaped composite parts. Many methods of increasing tack have been investigated. As a result of such investigations, numerous new resin monomers have been introduced into the market. However, despite initial expectations, the use of these new and often higher cost monomers has not resulted in the increase in tack desired in composites, especially if the resin system contains particles as part of the formulations.
Because of the requirements of the aerospace industry, most of the tackified prepregs described will have damage tolerance of greater than 138 MPa (20 Ksi), and in most cases greater than 172 MPa (25 Ksi) at 6.7 J/mm (1500 in-lb/in) impact level. In order to attain these damage tolerance levels, the rubber particle tackifying aid will be most advantageously used in combination with other particles, in the examples described either bismaleimide or engineering thermoplastic particles. The tackifier works by holding the resin on the prepreg surface.
Furthermore, engineered thermoplastics used to increase damage tolerance of the cured crosslinked network when formulating the toughening resins often present the problem in which the use of thermoplastic as interleaf tougheners causes poor tack retention on the resin surface. Additionally, the use of discrete, rigid thermoplastic particles causes an adverse effect in tack retention after the resin is transferred to the prepregs. "Resin soak-in", a phenomenon that occurs when the liquid resin drains through the fiber or the particles located on the surface of the prepreg, causes the tack to diminish significantly.
Soluble, reactive elastomers, have been used in epoxy adhesives, however, the addition of soluble elastomers to epoxies for use in fiber-reinforced composites results in a decrease in modulus, strength, and use temperature.
In copending U.S. application Ser. No. 08/126,012 filed on Sep. 24, 1993, which is a continuation of application Ser. No. 07/756,001 (now abandoned), which is a continuation-in-part of application Ser. No. 07/738,006, filed on Jul. 30, 1991, now abandoned, all of which applications are herein incorporated by reference, successful approaches to toughening epoxy resin matrices with respect to bismaleimides were disclosed. Nevertheless, in the past, there has been little success directed to toughened (impact resistant) epoxy resin matrices having superior tack, which as a practical sense, is of great importance in the industry because the industry is moving to more complex shaped parts.
In addressing tack, the formulation of, for example, bismaleimide resin using melt processing and solvent processing to produce the resin should be considered. Solvent processing offers the advantage of adjustable tack and viscosity by the use of the solvent. However, this process creates environmental hazards in production and handling and also requires sophisticated void control during the cure. In contrast, melt processing is solvent free, and can be achieved by two methods: a completely homogeneous resin melt system, or a slurry mixing system. The homogeneous system is highly viscous, however, it usually results in prepregs with inadequate tack and drape.
Inclusion of soft aliphatic segments into the backbone of the monomer systems has been attempted to improve tack. However, this procedure enhances the tack only to a minor extent by lowering the T.sub.g and the mechanical performance of the cured properties. Therefore, it is desirable to develop other effective tacky and tough resin systems which is able to maintain ultimate T.sub.g and the mechanical characteristics in the cured parts.